The negative water potential in a flower just before it is cut causes the columns of water in its xylem to snap into the flower stem and away from the cut surface. This means that there will be air bubbles in the xylem, inhibiting water uptake by the flowers, followed by wilting and death. Re-cutting with the original cut surface under water means that water can be pulled up into the xylem this time, leaving a continuous column of water all the way to the new cut surface. This will allow the cut flowers to absorb water from their vase, avoiding wilting.
Work Step by Step
Review Concepts 36.3 and 36.4 and use your imagination to see what occurs when flower stems are cutting in air versus in water.